I. Pseudorabies Disease
Pseudorabies, a highly contagious disease of swine and other livestock, such as cattle sheep and goats, is caused by Herpesvirus suis (hereinafter "pseudorabies virus" or "PRV"). In swine, the disease causes respiratory illness and encephalitis which may progress to death. Other common consequences of infection in swine are abortions neonatal demise, reduced litter size and slower growth rates. In other livestock, most notably cattle, PRV infection almost invariably proceeds to a lethal encephalitis.
Pseudorabies has become a major threat and cause of economic loss to the swine industry throughout the world. There is also considerable alarm over the spread of pseudorabies to cattle and other farm animals. Within the last ten years, economic losses have escalated because of the emergence of more virulent strains of PRV and the widespread dissemination of the disease. Today, it is estimated that 8.0% of the 80 million hogs on farms in the United States are infected in comparison to less than 0.8% a decade ago.
The clinical symptoms and consequences of PRV infection may be moderated or prevented by the use of vaccines comprising either killed or modified live, i.e., attenuated strains of PRV. However, most existing vaccines have failed to control the spread of pseudorabies disease because of a unique biological property of PRV and the other alpha-herpesviruses, such as herpes simplex virus types 1 and 2, varicella-zoster, infectious bovine rhinotracheitis virus, marmoset herpesvirus, and equine herpesvirus type 1.
More specifically, alpha-herpesviruses have the special ability to enter into a dormant state in neural tissues. That is, as an animal recovers from the initial generalized infection alpha-herpesviruses retreat to portions of the nervous system where they become quiescent and impervious to the body's immune defenses. This dormant infection, i.e., latency, may be unexpectedly reactivated, resulting in recrudescence of disease or in a contagious condition known as the carrier state, wherein the infected animal shows no outward symptoms of the disease but can transmit or "shed" infectious alpha-herpesviruses intermittently so as to cause the spread of infection and epidemic outbreaks.